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Old 07-12-2012, 05:48 PM   #1
Milkjug OP
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Me me, I an airhead

Making progress in my life meant getting a '75 r90/6 with 69k on it. I have wanted one as long as I can remember and have put 3k on it since I bought it. I had a few issues with the electronics but solved them and I'm back on the road. The thing idles kind of lopey and doesn't seem right I cannot get it to smooth out no matter what I do. I have it set at 1200. It also vibrates pretty bad through the powerband but is smooth above 3k to about 4.5k. Plugs are new and it's had a top end recently. I'm going to check compression and change wires and possibly rebuild carbs and get new cables. The bike is really clean and I will make sure it stays that way. I'll post pics soon.
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Old 07-12-2012, 06:15 PM   #2
ghostdncr
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Congratulations on your find! I'm still looking for my first one. All that vibration, isn't that often caused by having the carbs out of synch?
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Old 07-12-2012, 08:40 PM   #3
Pica Hudsonia
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I'm pretty inexperienced myself with these things, but I've read LOTS of tech articles online from the airhead gurus. It is often suggested that the carb sync. is important for smooth idling, and so is very careful valve adjustment. Some say if the valve clearances are not precisely .1 and .2 mm, that's okay, but the left and right sides must match exactly, so you should do both intake valves at the same time, then both exhausts, looking for the same feel from the gages as you drag them through the gaps.
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Old 07-13-2012, 12:03 AM   #4
Milkjug OP
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I rode the bike tonight and got a lot of vibration out, but not all ofcourse. My idle is lopey, rough and annoys me as that is by the book spot on sync'd. Mixtures are fine at a hair less than 1 turn out so that leaves ignition. The p/o put BMW "eta" blue plug wires that are for the low revving eta engine and the right carb is less responsive to adjustment than the right if that makes sense. I heard another R90 idle and it sounded very even, I fear that the uneven, unpredictable state of running will hurt the bottom end. I also understand I'm dealing with a nearly 40 year old machine that wasn't used much in the last few years. And words of wisdom?
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Old 07-13-2012, 12:24 AM   #5
sharkojoints
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1. Check and adjust valve clearances

2. Check and adjust fuel mixture

3. Synch the carbs, at idle (850 ish for your motor?) and at about 3500 - 4000 rpm

4. Road test and repeat if necessary

You might also find you need to adjust float height or replace carb diaphragms.

All the info you need is either here:

http://w6rec.com/duane/bmw/

or here:

http://bmwmotorcycletech.info/techni...icles-list.htm

These bikes are simple, and respond to a careful, methodical approach. If it's not working right after you've fiddled with it then you've possibly not been careful and methodical enough!

It feels good when you finally set it up right!
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Old 07-13-2012, 04:55 AM   #6
ghostdncr
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I recall a fellow posting awhile back with a similar issue and eventually he realized his bike had two completely different carbs on it. Can't find the post (it may have even been on another site) or remember the details, but the PO had replaced one of his R80 carbs with one from an R65 or some such. That would cause some imbalance, right?
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Old 07-13-2012, 06:25 AM   #7
disston
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All very good pointers so far. Which one will straighten out your ride? I like the one about unmatched carbs. These bikes can have gone thru a lot before they get much attention. I've seen some Beemers that I thought were in bad enough condition to condemn but they were still running.

If that mileage is correct, almost always suspect, but if close then the pointer I have for you is not too likely. Still it is a mater that needs close scrutiny and care in adjustment. I'm talking about the ignition. Remember this old saying, "Any time the carbs are suspect check the ignition." The points ignitions our bikes came with are not plug and play. They get adjustments and special care of several of their components. There's the cam tip, advance unit, condenser, coils, spark plugs and wiring, two kinds, primary and secondary. The adjustment of the points involves two particulars, both are needed, the dwell or point gap and the timing (we'll start with timing at idle but there is more). Have you learned to install and adjust ignition points? Have you adjusted the point gap on a new set of points? Is the timing correct at idle and do you have the timing light to check timing at full advance?

If you have the timing light then my suggestion is to look at the timing and look for a ghost in the image. You are looking for a dual image of the timing, firing of the plugs, in the timing window. My 1975 R90/6 had a lopey idle and poor performance for years till I fixed the double timing image.

There is more info on this mater and the basic solution is explained on the Duane Ausherman site that somebody else has already given you the address for. Do check into this. If more explanation is needed I can write more.

Charlie

ps; tell us the #s on the sides of the Bing carbs mounted on the bike. They should be 64/32/??
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Old 07-13-2012, 06:44 AM   #8
montmil
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First, as noted in previous responses, work your way through the tune up basics of valve lash and ignition before jumping on the Bings. You may also need to confirm quality of spark plug caps, leads and coil.

The Airheads all seem to have a cruise rpm where the engine has more vibrations than at other points in the rev range. Even with perfect carb synch, it's a bit of the nature of the beast. Adjust your cruise speed up or down and you'll get a smoother ride.

Enjoy your new BMW.

My favorite carb balancing act is attached.

http://www.bmwr65.org/htdocs/yabbfil...nchro_Word.pdf
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Old 07-13-2012, 07:07 AM   #9
Bill Harris
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Quote:
It also vibrates pretty bad through the powerband but is smooth above 3k to about 4.5k.
That is pretty much the best RPM range. Below 3000upm is considered lugging the engine. You can gently run the engine up from 2500 (or even 2000) with a very light throttle, but if you give it too much gas the engine won't be happy. Remember, it is a 2-cylinder engine and you'll feel the individual power pulses at lower rpm, and feel them more on the newer bikes with light flywheels.
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Old 07-13-2012, 07:13 AM   #10
ghostdncr
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Quote:
Originally Posted by montmil View Post
My favorite carb balancing act is attached.

http://www.bmwr65.org/htdocs/yabbfil...nchro_Word.pdf
Thanks for sharing the link, Monte. Can't imagine anyone having questions about the procedure after reading this!
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Old 07-13-2012, 07:22 AM   #11
ignatz72
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You will be wasting your time trying to balance the carbs at anything above 2000 RPM, the 'CV' part of these carbs takes care of that.

Instead, worry about idle (mixture and throttle plate stops) and just off-idle (1500-ish RPM, via the throttle cable ferrules).

That is, if all else checks out first. The order of precedence I follow for tuning is new spark plugs, timing, valves, carbs. (Really it's timing, valves, carbs but spark plugs are cheap and easy. Plus new plugs will help reading when tuning carbs)
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Old 07-13-2012, 07:34 AM   #12
Chad M
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A vacuum leak or a damaged diaphragm are two other areas that can cause an uneven idle and difficulty in tuning.
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Old 07-13-2012, 09:30 AM   #13
Milkjug OP
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Thanks for the responses gents. The bings have coarse threaded adjustments which makes them sensitive. I will double check for runout at the cam nose with a dial indicator, I just set points and I'm running a dyna booster, valves are set, I get 44mpg, running fresh brad penn 20w50 with a napa filter (which is very high quality). I'm gonna change wires, check for that damn ghost and if he's spooking my timing. I agree with the guy who said if you have a carb issue, check ignition. My mileage is correct by the way. Carbs have matching number plates and same throat size, but I don't know the condition of the diaphragms. Could it need a timing set??? Hope not and how would I know?
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Old 07-13-2012, 09:55 AM   #14
disston
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Tiny, tiny holes in the diaphragms are really hard to see. You hold them up to the sunlight and stretch them with the fingers. By the time you find any holes they are really running bad. Not a source of minor problems. Think, it runs on one cylinder type stuff. If you don't know the age of the diaphragms it's usually a good idea to change them.

"timing set" if you mean points and condenser maybe. But I think you said it has some electronic box. Oh yeah, a Dyna Booster. So the points are only a switch.

Get some proper plug wires. The plugs should be non-resister and the caps should be 1000 ohms. You can get the proper stuff from any body that sells BMW Airhead parts. Try Ted Porter in California. He will have the cheaper NGK caps and plugs;

http://www.beemershop.com/
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Old 07-13-2012, 12:03 PM   #15
Bill Harris
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Quote:
Could it need a timing set???
New points? Old points? So you just set the points gap without adjusting/checking the timing? Not good. New points, just be the certain, and check and adjust the timing. Current BMW points are Chinese and suspect, try and get Noris (brand) points.

And be sure to use a dollop of Points Cam Lubricant.

Then we'll be sure.
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